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International audience; The Demerara Plateau is a marginal plateau which forms a bathymetric relief on the sea floor. Here, contourite deposits have been studied in detail, following the recent discovery of contourite sequences likely related to the bottom currents and linked both to contour current and peculiar sea-floor morphology. A chronostratigraphic framework, based on δ18O relative variations and palaeomagnetic events in sediment cores allows correlating sediment processes to current intensity changes and major climate phases (glacial or interglacial). The studied sediments are enriched in glauconitic grains. In addition, the glauconite mineralogical maturity can easily correlate to low sedimentation rate and slightly energetic bottom currents on the seafloor. Based on these data and using the glauconitic authigenic mineral as proxy for inferring the degree of winnowing at the sediment-water interface, we might put forward the hypothesis that the intensity of NADW is higher during the glacial stages and lower during interglacial periods.

International audience; River-dominated ocean margins (RiOMars) are areas of high productivity, rapid sediment deposition, and in-tense recycling. The Gulf of Lions, located in the North occidental Mediterranean Sea, is no exception to thisgeneral rule and is under the prevalent influence of the Rhône River. One particularity of this continental shelf is that it is also fed by several small coastal rivers whose inputs actively contribute to the coastal dynamic and export to the deep-sea. Our objective was to gauge the influence of coastal rivers on the quality and reactivity of the organic matter delivered to the continental shelf as these properties control two important functions of coastal areas: benthic productivity and carbon sequestration. We chose for this study, the Têt River, which is typical of coastal Mediterranean rivers with a torrential regime. Sediment cores were sampled five times over a one-year period on four stations located along the dominant trajectory of the Têt River plume. The biogeochemical characteristics of the surface sediments were analysed to highlight spatio-temporal trends in the sedimentary organic matter composition and to identify the major environmental factors controlling its reactivity. Nutrient and dissolved oxygen fluxes at the water-sediment interface were furthermore measured at two stations using ex-situ whole core incubations.Results from this study highlight the seasonal influence of riverine versus autochthonous productions on thequality of sedimentary organic matter in the vicinity of the river mouth. Farther on the mid shelf, a terrestrial imprint is still visible, but the temporal signal is blurred as a consequence of strong mixing during frequent episodes of resuspension/deposition. Organic matter is efficiently remineralised in this system with total oxygen uptake (TOU) rates comparable to those measured off the Rhône River. Finally, organic matter quality and reactivity are also constrained in this system by physical forcings, which promote remineralisation and advection of sediments.

The TEX 86 H paleothermometer based on isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) has widely been applied in various marine settings to reconstruct past sea surface temperatures (SSTs). However, it still remains uncertain how well this proxy reconstructs annual mean SSTs. Here, we assess environmental factors governing the TEX 86 H paleothermometer in the Mediterranean Sea, by studying the distribution of isoGDGTs in surface sediments, suspended particulate matter (SPM), and two sediment cores. A redundancy analysis using the fractional abundance of the six major isoGDGTs indicates that the sedimentary isoGDGTs are mostly influenced by three environmental factors explaining a large part (74%) of the variance in isoGDGT distribution. In order of decreasing significance, these factors are annual mean SST, continental organic matter input as indicated by the BIT index, and water depth. However, when considering only the four isoGDGTs that are used for the TEX 86 H proxy, water depth is the most significant parameter, explaining 63% of the variance. Indeed, a strong positive relationship between water depth and TEX 86 H is observed in both surface sediments and SPM from the Mediterranean Sea. This is driven by an increase in fractional abundances of GDGT-2 and crenarchaeol regio-isomer and a decrease in the fractional abundances of GDGT-1 and GDGT-3 with increasing water depth, leading to a bias to higher temperatures of TEX 86 H in deep-water surface sediments. The fact that the water-depth trend is also apparent in SPM suggests that this change might be due to a change in thaumarchaeotal community thriving below surface mixed-layer waters and that this signal is, at least partly, incorporated into sedimentary isoGDGTs. Interestingly, surface-sediment TEX 86 H values from >1000 m water depth do not show a correlation with water depth anymore and instead are correlated to annual mean SSTs. A composite deep-water TEX 86 H dataset of surface sediments from both the Mediterranean Sea and the Red Sea, interconnected regional restricted basins with relatively high bottom-water temperatures and high salinity, forms a distinctive correlation line, statistically distinct from that of the general global correlation. Application of this correlation on two sedimentary records from the western Mediterranean Sea covering the last deglaciation yields SSTs nearly identical to those obtained with the U 37 K ′ paleothermometer, whereas the global calibration substantially overestimates SSTs. Our results show that the warm bias of the TEX 86 H proxy in the Mediterranean Sea is not due to seasonality, as previously suggested. Further research is needed to elucidate the mechanism behind the strong water depth trend of TEX 86 H in the Mediterranean Sea which is not apparent in open ocean settings.

Despite the ecologic and economical importance of coastal areas, the neurotoxic bioaccumulable monomethylmercury (MMHg) fluxes within the ocean margins and exchanges with the open sea remain unassessed. The aim of this paper is to address the questions of the abundance, distribution, production and exchanges of methylated mercury species (MeHgT), including MMHg and dimethylmercury (DMHg), in the waters, atmosphere and sediments of the Northwestern Mediterranean margin including the Rhone River delta, the continental shelf and its slope (Gulf of Lions) and the adjacent open sea (North Gyre). Concentrations of MeHgT ranged from

International audience; We tested the applicability of the BIT (branched and isoprenoid tetraether) index as a proxy for palaeoflood events in the river-dominated continental margin of the Gulf of Lions (NW Mediterranean). We compared the concentrations of branched glycerol dialkyl glycerol tetraethers (br GDGTs) and crenarchaeol in suspended particulate matter (SPM) collected downstream in the Rhône River, as well as in surface sediments and a ca. 8m piston core from the Rhône prodelta. The core covered the last 400 yr, with four distinct intervals recording the river influence under natural and man-induced shifts in four main channels of the river mouth (Bras de Fer, Grand Rhône, Pégoulier,and Roustan). The results indicate that there are mixed sources of br GDGTs and crenarchaeol in the prodelta, complicating applicationof the BIT index as an indicator of continental organic carbon input and thus as a palaeoflood proxy. However, the sedimentary BIT record for the period when continental material was delivered by the river more directly to the core site (Roustan phase;1892 to present) mimics the historical palaeoflood record. This shows the potential of the BIT index as a palaeoflood proxy, provided that the delivery route of the continental material by rivers to the core sites remains constant over time. The study also highlights the idea that shifts in river channels should be taken into account for the use of the BIT index as a palaeoflood proxy.

The Gulf of Lions is a river-dominated ocean margin that receives high loads of nutrients and particulate matter from the Rhône River but most particulate materials settle rapidly on the nearshore seafloor. One question is raised on the fate of these large quantities of organic carbon delivered by the river to the coastal marine environment. Surface sediments (0–0.5 cm) were collected in the Rhône prodelta and its adjacent shelf during a period of low river discharge (April 2007, 16 stations). The sources, distribution and lability of sedimentary organic matter were examined using bulk (organic carbon, total nitrogen, stable carbon isotope ratios, and grain size) and molecular-level (pigments, amino acids, fatty acids, and δ13C of individual fatty acids) analyses. Our results confirmed previous observations of a southwestward Rhodanian imprint in the nearshore sediments, with 97% of terrigenous inputs of organic matter near the river mouth. Isotopic values of bulk organic carbon, as well as fatty acid biomarkers and compound-specific δ13C signatures of most fatty acids clearly indicate that the Rhône inputs consist of a mixture of organic matter (OM) from different origins with a strong contribution from terrestrial sources (soil and plant debris), and a smaller input from freshwater microalgae, mostly diatoms. The influence of the Rhône River was prominent within the first ten kilometers, but may still be observed on the outer shelf (~21 km) as indicated by the occurrence of long chain fatty acids, which are derived from vascular plants, and their δ13C signatures. In the proximal prodelta, bacteria-specific fatty acids were abundant (1.65 mg g−1 OC at the mouth site) and were relatively depleted in δ13C confirming that bacteria mostly utilize land-derived OM. In the shelf area, the inputs of marine OM and its predominant utilization by the bacteria was confirmed, but the coupling between the pelagic and the benthic compartments appeared limited at this period of the year. Overall, degradation indexes based on amino acids (Dauwe's degradation index) and pigments (ratio of intact chlorophyll-a to the sum of chlorophyll-a + phaeopigment-a), as well as isotopic enrichment of source-specific fatty acids reveal an offshore gradient of OM decay reflecting the rapid deposition of the terrestrial material in the prodelta, the low mixing with OM deriving from marine sources and the efficient degradation of the OM. The OM delivered by the Rhône is relatively labile based on the intermediary value of Dauwe's degradation index, the high proportion of bio-available nitrogen and the occurrence of polyunsaturated fatty acids. Deltaic sediments off the Rhône River should thus be of sufficiently high nutritional quality to sustain dense macrofaunal communities.

River deltas are particularly important in the marine carbon cycle as they represent the transition between terrestrial and marine carbon: linked to major burial zones, they are reprocessing zones where large carbon fluxes can be mineralized. In order to estimate this mineralization, sediment oxygen uptake rates were measured in continental shelf sediments and river prodelta over different seasons near the outlet of the Rhône River in the Mediterranean Sea. On a selected set of 10 stations in the river prodelta and nearby continental shelf, in situ diffusive oxygen uptake (DOU) and laboratory total oxygen uptake (TOU) measurements were performed in early spring and summer 2007 and late spring and winter 2008. In and ex situ DOU did not show any significant differences except for shallowest organic rich stations. Sediment DOU rates show highest values concentrated close to the river mouth (approx. 20 mmol O2 m−2 d−1) and decrease offshore to values around 4.5 mmol O2 m−2 d−1 with lowest gradients in a south west direction linked to the preferential transport of the finest riverine material. Core incubation TOU showed the same spatial pattern with an averaged TOU/DOU ratio of 1.2±0.4. Temporal variations of sediment DOU over different sampling periods, spring summer and late fall, were limited and benthic mineralization rates presented a stable spatial pattern. A flood of the Rhône River occurred in June 2008 and delivered up to 30 cm of new soft muddy deposit. Immediately after this flood, sediment DOU rates close to the river mouth dropped from around 15–20 mmol O2 m−2 d−1 to values close to 10 mmol O2 m−2 d−1, in response to the deposition near the river outlet of low reactivity organic matter associated to fine material. Six months later, the oxygen distribution had relaxed back to its initial stage: the initial spatial distribution was found again underlining the active microbial degradation rates involved and the role of further deposits. These results highlight the immediate response of the sediment oxygen system to flood deposit and the rapid relaxation of this system towards its initial state (6 months or less) potentially linked to further deposits of reactive material.

International audience; Expanded marine Holocene archives are relatively scarce in the Mediterranean Sea because most of the sediments were trapped in catchment areas during this period. Mud belts are the most suitable targets to access expanded Holocene records. These sedimentary bodies represent excellent archives for the study of sea–land interactions and notably the impact of the hydrological activity on sediment accumulation. We retrieved a 7.2 m long sediment core from the Rhône mud belt in the Gulf of Lions in an area where the average accumulation rate is ca. 0.70 m 1000 yr−1. This core thus provides a continuous and high-resolution record of the last 10 ka cal BP. A multiproxy dataset (XRF core scan, 14 C dates, grain size and organic-matter analysis) combined with seismic stratigraphic analysis was used to document decadal to centennial changes in the Rhône hydrological activity. Our results show that (1) the early Holocene was characterized by high sediment delivery likely indicative of local intense (but short-duration) rainfall events, (2) important sediment delivery around 7 ka cal BP presumably related to increased river flux, (3) a progressive increase in continental/marine input during the mid-Holocene despite increased distance from river outlets due to sea-level rise possibly related to higher atmospheric humidity caused by the southward migration of the storm tracks in the North Atlantic, (4) multidecadal to centennial humid events took place in the late Holocene. Some of these events correspond to the cold periods identified in the North Atlantic (Little Ice Age, LIA; Dark Ages Cold Period) and also coincide with time intervals of major floods in the northern Alps. Other humid events are also observed during relatively warm periods (Roman Humid Period and Medieval Climate Anomaly).

In this study, we investigated soils and river suspended particulate matter (SPM) collected in the Rhône and its tributary basins as well as marine surface sediments taken in the Rhône prodelta (Gulf of Lions, NW Mediterranean). Thereby, we traced the signal of branched glycerol dialkyl glycerol tetraethers (brGDGTs) from the source to sink via the Rhône River and its tributaries and identified sources of brGDGTs in rivers and marine sediments. Soil pH rather than the mean annual air temperature (MAAT) explains most of the observed variances of the brGDGT distribution in our soil dataset. The observed changes in the distribution of brGDGTs in the river SPM indicate that brGDGTs brought by the river to the sea are primarily derived from the lower Rhône and its tributary soils, even though in situ production in the river itself cannot be excluded. In marine surface sediments, it appears that the input of riverine brGDGTs is the primary source of brGDGTs in the Rhône prodelta, although the brGDGT composition may be further modified by the in situ production in the marine environment. More work is required to assess fully whether brGDGTs can be used to reconstruct the terrestrial paleoenvironmental changes using marine sediment cores taken in the Rhône prodelta close to the river mouth.

International audience; Major rivers transfer high loads of continental particulate organic matter to deltaic environments, where the impact on the biogeochemistry and productivity of coastal sediments depends on the sources and lability of these inputs. Our aims are to provide new insights into the reactivity of riverine inputs in coastal environments and to delineate the parameters controlling their fate in these dynamic systems. Sediment cores were collected from a Mediterranean deltaic system (the Rhône prodelta and its adjacent shelf) during a period of moderate river discharge (Spring 2007) and analyzed for their fatty acid composition. Sediment properties were also described using biochemical bulk analyses (organic carbon and lipids), granulometry and profiles of redox potential. Based on the bulk compositional changes and hierarchical clustering of the fatty acid biomarkers, we determined the principal sources of organic matter and their spatial distribution. Fatty acids were primarily of continental origin in the prodelta, shifting to a higher contribution from marine sources in the shelf area. Fatty acids derived from both continental and marine sources were efficiently degraded in the sediments by microbial decay processes in the upper oxic layer. Degradation rate constants calculated from the down-core decreases in concentrations indicate that fatty acid degradation was enhanced in sediments influenced by the Rhône River. The most important parameters affecting fatty acid preservation were the inherent stability of individual molecular components and their physical association with mineral matrices, the source and freshness of the inputs, and the depositional environment (redox condition, accumulation rates).

International audience; The consequences of changes in the water flow of the Rhône River on surface sediment characteristics and benthic macrofauna composition were assessed within 3 distinct areas: (1) the delta front, (2) the prodelta, and (3) the distal zone. Five stations were sampled during or closely after: (1) an oceanic flood (April 2007), (2) a generalized flood (May 2008), (3) a Cevenol flood (December 2008), and (4) a dry period (July 2011). Measurements of sediment characteristics included granulometry (D0.5), bulk descriptors of sedimentary organics (OC, TN and THAA), descriptors of labile components of sedimentary organics (chloropigments, EHAA), and both descriptors of origin (Chl-b/Chl-a, C/N) and lability (Chl-a/(Chl-a+Phaeo-a), EHAA/THAA) of sedimentary organics. Sediment Profile Images were collected during April 2007, May 2008 and July 2011. Temporal changes in both sedimentary organics and benthic macrofauna were more important in the delta front and the prodelta than in the distal zone. Bulk characteristics of sedimentary organics presented decreasing inshore/offshore gradients during both April 2007 and July 2011 but not during May and December 2008. There were significant temporal changes in EHAA/THAA at all stations. Changes in benthic macrofauna composition differed between: (1) the delta front and the prodelta, and (2) the distal zone. In the former area, the dry period was associated with establishing a mature community characterized by high abundances and species richness. The best description of spatiotemporal changes in benthic macrofauna composition by surface sediment characteristics was obtained using D0.5, Chl-b/Chl-a, Chl-a/(Chl-a+Phaeo-a) and EHAA, which supports the role of the quality of sedimentary organics in controlling benthic macrofauna composition.

A detailed sedimentological and geochemical investigation was performed on slope sediments collected from the Gulf of Lions and Catalonian margins and from the northern Balearic margin. The study was based on cores, about 50 cm long. Various physiographic sites (canyons, open slopes) were selected according to the available sediment accumulation rates obtained with the Pb-210 geochronological method. At a short time scale, various parts of the slope were characterized by muddy hemipelagic sediment. Sedimentological data suggested that deposition was continuous. Total organic carbon and its hydrolysable fraction decreased downcore. Significant increase of C/N ratio downcore resulted from an early diagenetic process in response to the burial effect. However in other parts, the values of the C/N ratio were found to be irregular and maxima could frequently be related to both higher sand content and relict coastal remains derived from erosion and reworking of outcropping deposits of the last glacial maximum (lowstand system tract). Such tracts crop out over much of the shelf slope break seafloor. They provide various biological and lithological indicators: relatively well-preserved Miliolidea, coastal or lagoonal foraminifera (Ammonia beccarii, Elphidilan crispum), Characea oogons, oxidized beach-rock debris, gypsum, previously mature glauconite (with cracks) and coarse vegetal debris. Throughout the sections, the degree of diagenetic overprinting of the organic matter frequently showed a positive correlation with the abundance of some coastal indicators supporting the hypothesis of sedimentation induced predominantly by gravity These downslope accumulation may correspond to 10-40 % of the total deposit. The gravity induced transfers have been shown to be negligible in the sediment record of the northern Balearic margin. It is suggested that a reduced sedimentation rate (half that observed in the Gulf of Lions) would be expressed by a higher stability of the slope. The consequence of low sedimentation would be early diagenetic solution/reprecipitation of carbonate near the redox transition layer., Une étude sédimentologique et géochimique détaillée des dépôts de pente du golfe du Lion, de la Catalogne et du Nord des Baléares à partir de carottes courtes ne dépassant pas 50 cm a été effectuée. Les sites présentés (canyons, interfluves) ont été choisis en fonction de l’existence de taux de sédimentation mesurés à l’aide de la méthode au 210Pb. À cette échelle de temps réduite, plusieurs régions de la pente sont caractérisées par une accumulation de vases hémipélagiques. Les observations sédimentologiques suggèrent un processus continu de la sédimentation. Le carbone organique total ainsi que sa fraction hydrolysable tendent à diminuer du haut vers le bas de la carotte ; à l’inverse une nette augmentation du rapport C/N est observée. Cette augmentation traduit les effets d’une diagenèse précoce conséquente de l’enfouissement. Cependant, dans certains sites, le profil vertical du C/N est irrégulier et les maxima peuvent être souvent corrélés à la fois avec des teneurs en sable plus élevées et avec l’abondance de débris issus de l’érosion et du remaniement des dépôts littoraux du dernier bas niveau glaciaire. Ces derniers qui tendent à affleurer à proximité de la rupture de pente entre le plateau et la pente sont reconnus grâce à plusieurs marqueurs littoraux : Miliolidea assez bien préservés, foraminifères côtiers ou lagunaires (Ammonia beccarrii, Elphidium crispum), oogones de Characées, fragments oxydés de grès de plage, gypse, glauconie évoluée et débris grossiers de végétaux. La fréquente association de ces indicateurs côtiers avec le degré avancé de l’évolution diagénétique correspond à des épisodes où l’accumulation gravitaire joue un rôle important pouvant représenter de 10 à 40 % du sédiment total. Sur la marge septentrionale des Baléares, ces processus gravitaires paraissent avoir joué un rôle négligeable. Il est suggéré que la vitesse de sédimentation réduite correspond à une plus grande stabilité de la pente. En conséquence de cette lenteur de la sédimentation, les carbonates sont l’objet d’une diagenèse précoce avec des processus de dissolution et de reprécipitation à proximité de la limite d’oxydoréduction.

The present work presents improvements to the existing methods for extraction of carbohydrates with water. In order to test the effects of extraction factors on the yield of carbohydrates, extractions of water-soluble carbohydrates were performed with two contrasting samples (suspended particulate organic matter from the high turbidity zone of the Gironde estuary and sedimentary organic matter from the Arcachon lagoon) under different experimental conditions. Results show that: (i) boiling water applied for 30 min yielded the best result for water-soluble carbohydrates in both samples; (ii) sonic disruption had a strong influence on the recovery rate of water-soluble carbohydrates - a steady yield was reached after a 3-min treatment; (iii) almost all the water-extractable carbohydrates are removed after the third extraction. Consequently, a general method is proposed to improve the extraction of water-extractable carbohydrates from particulate matter., Sur la base de deux échantillons organiques de nature opposée (matières en suspension de la zone de maximum de turbidité de l'estuaire de la Gironde et sédiments du bassin d'Arcachon), on a pu évaluer les effets de différents facteurs d'extraction sur la méthode d'extraction des sucres par l'eau. Ainsi, le rendement d'extraction des sucres solubles devient optimum (i) lorsque l'extraction est conduite à 100 °C et dans une période minimale de 30 min ; (ii) lors de l'application d'un traitement aux ultrasons de 3 min ; (iii) après trois extractions successives. Une optimisation de l'extraction des sucres solubles est alors proposée en vue de l'harmonisation des méthodes d'extraction.

International audience; On March 11th, 2011 the Mw 9.0 2011 Tohoku-Oki earthquake resulted in a tsunami which caused major devastation in coastal areas. Along the Japanese NE coast, tsunami waves reached maximum run-ups of 40 m, and travelled kilometers inland. Whereas devastation was clearly visible on land, underwater impact is much more difficult to assess. Here, we report unexpected results obtained during a research cruise targeting the seafloor off Shimokita (NE Japan), shortly (five months) after the disaster. The geography of the studied area is characterized by smooth coastline and a gradually descending shelf slope. Although high-energy tsunami waves caused major sediment reworking in shallow-water environments, investigated shelf ecosystems were characterized by surprisingly high benthic diversity and showed no evidence of mass mortality. Conversely, just beyond the shelf break, the benthic ecosystem was dominated by a low-diversity, opportunistic fauna indicating ongoing colonization of massive sand-bed deposits.

13 pages, 8 figures, 6 tables, 1 appendix supplementary datahttps://doi.org/10.1016/j.chemgeo.2014.04.015, Continental margins receive natural and anthropogenic trace elements (TEs) from direct atmospheric deposition of aerosols onto the sea surface and from advection of riverine suspended particles and/or resuspended sediments from the continental shelf/slope. When the margin is incised by submarine canyons, as for example in the Northwestern Mediterranean Sea, most of these particles are preferentially transferred via these topographic features towards their final repositories in the abyssal plain. The Gulf of Lions (GoL) shelf receives the largest particulate riverine input to the Western Mediterranean, with its associated chemical contaminants originating from the industrialized and urbanized Rhone Valley. Sediment samples (grabs, cores and moored traps) collected in the Cap de Creus (CdC) Canyon and its adjacent areas at the Southwestern exit of the GoL were analyzed to explore the origin, dispersion, transfer and accumulation of a suite of TEs (Ag, Cd, Co, Cr, Cu, Ni, Pb, Zn and V) from the GoL shelf to the adjacent continental rise. Distributions of Cu, Cr, Ni, Pb, Zn and V in the surface sediments of the shelf confirm their terrigenous origin in association with clay minerals, whereas Ag and Cd are more associated with organic matter (OM). All these TEs are anthropogenically enriched in the Rhone prodelta sediments. Anthropogenic influence remains clearly discernible in the GoL shelf surface sediments for Ag, Pb and Zn. Hydrodynamical resuspension and sorting of shelf sediments occur at the head of the CdC Canyon during dense shelf-water cascading events. During these events, the material collected in moored sediment traps contains a higher coarse carbonate fraction slightly impoverished in TEs compared to the clays of the nepheloid layer and the organically-rich particles deposited before and at the end of the cascading period. Upper and middle canyon sediments are characterized by high sedimentation rates (~0.2cmyr-1) of fine clay material. Conversely, sediments from the lower continental slope and rise exhibit low sedimentation rates (~0.06cmyr-1) and receive carbonaceous planktonic detritus from the water column. At the lower continental slope, coarse material includes foraminifers and pteropods, whereas at the continental rise finer planktonic-derived material is more abundant. Both in the CdC Canyon and in its adjacent lower continental slope/rise sediments, Co, Cu, Cr, Ni and V are associated with clay, whereas Ag, Cu and Pb are preferentially associated with OM. Cadmium, Cr, and Zn are also associated with OM in canyon sediments. Carbonaceous plankton appears to be especially efficient for scavenging Ag, whereas, Cr, V, Zn and Pb are diluted by biogenic carbonates. An authigenic Mn fraction is enriched with Co and Ni. Lead and Zn concentration levels and vertical profile patterns, along with Pb stable isotopic ratios, indicate that significant parts of Pb and Zn are of anthropogenic origin. A sediment chronology based on 210Pb dating reveals that Pb anthropization, mainly from gasoline additives, culminated between 1960 and 1980, being the current concentrations >40% lower than 30years ago. A similar distribution is observed for Zn, which originates mainly from combustion processes; but the reduction of Zn contamination amounts to only 20% during the same period. The largest anthropogenic Pb accumulation occurs in the middle part of CdC Canyon, with an inventory of 200μgcm-2. At the most distal part of the continental rise anthropogenic Pb accumulation within the first ~10cm below the surface sediment is estimated around 10μgcm-2, which is similar to the direct atmospheric deposition estimate. © 2014 Elsevier B.V., This research has been funded by Hermione project (www.euhermione.net), a collaborative project under the European Commission's Framework Seven Program (Contract No. 226354) and by the ONR (Contract No. N00014-04-1-0379)

International audience; Methylation of mercury (Hg) is the crucial process that controls Hg biomagnification along the aquatic food chains. Aquatic sediments are of particular interest because they constitute an essential reservoir where inorganic divalent Hg (HgII) is methylated. Methylmercury (MeHg) concentrations in sediments mainly result from the balance between methylation and demethylation reactions, two opposite natural processes primarily mediated by aquatic microorganisms. Thus, Hg availability and the activity of methylating microbial communities control the MeHg abundance in sediments. Consistently, some studies have reported a significant positive correlation between MeHg and HgII or total Hg (HgT), taken as a proxy for HgII, in aquatic sediments using enzyme-catalyzed methylation/demethylation mechanisms. By compiling 1,442 published and unpublished HgT–MeHg couples from lacustrine, riverine, estuarine and marine sediments covering various environmental conditions, from deep pristine abyssal to heavily contaminated riverine sediments, we show that a Michaelis–Menten type relationship is an appropriate model to relate the two parameters: MeHg = aHgT/(K m + HgT), with a = 0.277 ± 0.011 and K m = 188 ± 15 (R 2 = 0.70, p < 0.001). From K m variations, which depend on the various encountered environmental conditions, it appears that MeHg formation and accumulation are favoured in marine sediments compared to freshwater ones, and under oxic/suboxic conditions compared to anoxic ones, with redox potential and organic matter lability being the governing factors.

We present an overview of the organic matter distribution in surface sediments sampled on the Gulf of Lions margin from 600 to 2,300 m depth. Surface deposits are characterized by a “fluff” layer a few millimeters to a few centimeters thick. In the underlying fine-grained sediments, the carbonate content is high (30% DW), and the organic carbon content varies from 0.3 to 1% (DW). The highest organic carbon contents are along the axes and on the sides of the canyons (0.6-l%) and constitute midslope organic depocenters. The organic carbon content at 1,000 m depth depends on the width of the adjacent shelf. The lowest organic carbon value corresponds to the slightest declivity and, consequently, to a longer advective residence time. In the settling sides of the axial channel of the canyons, organic carbon concentrations decrease, although sedimentation rate is constant from the upper to the lower part of the slope. This results from an increasing residence time in the water column due to the advective transfer of organic material from epicontinental zone to upper and lower slope. In the lower axes and open slopes, increasing sedimentation rates, due to sedimentary reworking and turbidity currents, dilute organic carbon with mineral grains.

A significant fraction of the global carbon flux to the ocean occurs in River-dominated Ocean Margins (RiOMar) although large uncertainties remain in the cycle of organic matter (OM) in these systems. In particular, the OM sources and residence time have not been well clarified. Surface (0-1 cm) and sub-surface (3-4 cm) sediments and water column particles (bottom and intermediate depth) from the Rhone River delta system were collected in June 2005 and in April 2007 for a multi-proxy study. Lignin phenols, black carbon (BC), proto-kerogen/BC mixture, polycyclic aromatic hydrocarbons (PAHs), carbon stable isotope (delta C-13(OC)), and radiocarbon measurements (delta C-14(OC)) were carried out to characterize the source of sedimentary organic material and to address degradation and transport processes. The bulk OM in the prodelta sediment appears to have a predominantly modern terrigenous origin with a significant contribution of modern vascular C-3 plant detritus (Delta C-14(OC) = 27.9 parts per thousand, delta C-13(OC) = -27.4 parts per thousand). In contrast, the adjacent continental shelf, below the river plume, seems to be dominated by aged OM (Delta C-14(OC) = -400 parts per thousand, delta C-13(OC) = -24.2 parts per thousand), and shows no evidence of dilution and/or replacement by freshly produced marine carbon. Our data suggest an important contribution of black carbon (50% of OC) in the continental shelf sediments. Selective degradation processes occur along the main dispersal sediment system, promoting the loss of a modern terrestrial OM but also proto-kerogen-like OM. In addition, we hypothesize that during the transport across the shelf, a long term resuspension/deposition loop induces efficient long term degradation processes able to rework such refractory-like material until the OC is protected by the mineral matrix of particles.

As a main source of freshwater and particles, the Rhône River plays a major role in the biogeochemical cycle of organic carbon (OC) in the Mediterranean Sea. To better understand the origin of organic matter and the processes leading to its export to the coastal sea near the Rhône River, we measured radiocarbon (Δ14C) and stable carbon isotopes (δ13C) in the sediments of the delta, after density fractionation. In April 2007, 3 sites located along an offshore transect (A, C, and E) were sampled for surface sediments, and bulk sediment was separated into 4 fractions of different densities (2.5 g cm−3). In order to better understand the evolution of the OC along the transect, we investigated the OC sources and their evolution for each density fraction. Bulk OC shows a large increase in δ13C from −27.2′ nearshore to −24.5′ at offshore stations while Δ14C decreased from 59′ to −320′. The distribution of δ13C with density displayed a convex pattern at all stations. Except for fraction >2.5 g cm−3, δ13C increases by 2.5′ between stations A and E, indicating a loss of terrestrial signature. The distribution of Δ14C versus density had a concave pattern at all stations: at a single station, it showed a large heterogeneity with a difference of 500–600′ between the −3 fractions. A decrease in Δ14C of −400′ among the different density fractions was observed along the offshore transect. The density fraction >2.5 g cm−3 had less variability, with an average δ13C of −24.6 ± 0.4′ and Δ14C of −370 ± 115′. Several processes may explain this distribution: retention in the prodelta of large particles; mineralization of all fractions during the transport and deposition in the delta and shelf sediments; and dilution of terrestrial particles in continental shelf pool.

The temporal variations of the superficial (0–1 cm) sedimentary organic matter were studied at a depth of 26 m on the continental shelf of the Gulf of Lions (northwestern Mediterranean). The samples were analyzed for total organic carbon, coarse organic carbon (>40 μm), hydrolyzable organic carbon, nitrogen, total amino acids, total and individual sugars (HPLC), lignin-derived compounds (HPLC) and kerogens (acid-soluble, humic substances and humin). Seasonal variations of the organic compounds are related to the sedimentological, hydrodynamical and physico-chemical environmental conditions. The mean annual values of the different organic compounds analyzed show the low quantities and their evolved character at the sediment-water interface: 0.5% total org C (TOC) (d.w.), 0.049% N (d.w.), CN: 11.2, coarse org C (COQ: 62% of TOC, hydrolyzable org C: 45% TOC. The labile compounds represent a low percentage of the total organic matter (TOM), amino-acids: 12% of TOM and sugars: 5% of TOM. The relative proportions of soluble (humic) and insoluble kerogens (humin), respectively 6% and 94% of TOC are typical of a highly evolved organic matter. The large contribution of plant remains confirmed by the high proportion of COC, corresponds to a low proportion of humic substances and a high degree of condensation (HC = 1.3). The infrared spectroscopy determination of the functional groups of the humic substances permits us to confirm both autochtonous (marine) and allochtonous (terrestrial) sources of organic matter in the Tet prodeltaic accumulation area. Numerous functional groups identified reveal the fresh quality of the organic inputs at the sediment-water interface. Aliphaticity is well marked and nitrogenous compounds (1 and 2 amines) correspond to autochtonous production (in spring: phyto- and zoo-planktonic blooms in the euphotic zone; in summer: primary production under the thermocline and phytobenthic blooms). Sugars are well represented, but from two origins: phytoplanktonic, and issue from terrestrial high plants (ligno-cellulosic complexes). The aromatic functional groups are well identified (aromatic ether, phenol) as are the markers of a large contribution of terrestrial organic matter. The active hydrodynamical (resuspension) and biological, (consumption, bioturbation) events participate in the frequent reworking of the superficial deposits (muddy-silts) and, consequently, the active degradation (oxidization) of the organic matter. The more labile organic matter is rapidly consumed by the intense biological benthic activity. The organic C content is low (0.5% d.w.) except during the autumnal input (2.5% d.w.). This input (8 cm thick) disappears one month later, because of the resuspension induced by littoral currents. This same phenomenon induces during all the year the reworking of the first centimeter of the deposit. In terms of budget of the carbon cycle in the coastal area, this active hydrodynamical environment shows the supremacy of the carbon degradation over its burial.

International audience; The Mediterranean Sea constitutes a unique environment to study cold-seep ecosystems due to the presence of different geodynamic settings, from an active margin along the Mediterranean Ridge (MR) to a passive margin in the Nile Deep-Sea Fan (NDSF). We attempted to identify the structure of benthic communities associated with the Napoli and Amsterdam mud volcanoes (MVs) located on the MR and to establish the links between faunal distribution and environmental conditions at different spatial scales. Comparison between the 2 MVs revealed that the faunal distribution seemed to be mainly controlled by the characteristics of the microhabitats. On both geological structures, the variability between the different microhabitats was higher than the variability observed between replicates of the same microhabitat, and the distribution of macrofauna was apparently linked to gradients in physico-chemical conditions. The peripheral sites from Napoli were generally more oxygenated and harboured lower species richness than the active sites. The reduced sediment microhabitat from Amsterdam presented the highest methane concentrations and was mainly colonised by symbiont-bearing vesicomyid bivalves and heterotrophic dorvilleid polychaetes. Overall, a higher taxonomic diversity was observed on Napoli. Substratum type was hypothesised to be the second factor influencing faunal distribution. The results of this study highlight the high heterogeneity of faunal communities associated with seep ecosystems within this region and the need to pursue investigations at various spatial and temporal scales.

Several studies have shown the occurrence of major discrepancies between the most commonly used benthic biotic indices within the EU Water Framework Directive. Such studies have however failed to assess, which indices perform best because of the lack of an independent assessment of benthic habitat ecological quality. The present study aimed at filling this gap by comparing the use of several biotic indices derived from benthic macrofauna composition and one index derived from sediment profile imagery, based on the sampling of 16 stations along a gradient of sedimentary organic carbon concentrations (OC) in front of the mouth of the Rhone River. Five biotic benthic macrofauna indices (BQI, BQIES, AMBI, M-AMBI, and W) were computed together with the benthic habitat quality (BHQ) index, which is derived from sediment profile images (SPI). All sampled stations but one belonged to the same community as affected by a gradient of organic enrichment. BQI, BQIES and BHQ correlated significantly between each other's and with OC. Conversely, AMBI and W did not correlate with either BHQ or OC. As organic enrichment is known to constitute a major source of disturbance, this suggests that BQI and BQIES performed better than AMBI, M-AMBI and W in assessing the ecological quality of soft sediments in the studied area. Discrepancies between BQI and AMBI derived indices are discussed based on differences in the sensitivity/tolerance levels attributed to most dominant species by these two indices.

The dynamics of sediment-bound metals (Cs, Cu, Ni, Pb, Ti and Zn) were studied off the Tet River (western Gulf of Lion), a typical Mediterranean coastal river punctuated by short and violent flash-floods. Spatial and temporal sampling strategies were combined to elucidate the fate of these elements in response to both the riverine sediment input and the offshore transport of these sediments through hydrodynamics. Our results show the temporal entrapment of riverborne particles and associated metals, consecutively to a major flood event, in the nearshore sedimentary unit called prodelta. Here, deposition and resuspension mechanisms define a sedimentological cycle that could be followed completely in this study. In terms of speciation between reactive (labile) and residual fractions along the fluvio-deltaic continuum, our results show that Cu, Pb and Zn are the most labile (potentially mobile) metals in the river, in accordance with their contributions from anthropogenic sources. But in the marine surficial sediments, two main behaviours can be discriminated when compared to the riverine suspended particulate matter. While Pb and Zn depict rather a constant labile fraction, Cu is characterized by decreasing levels (up to 50% difference). In terms of environmental impact, these contrasting trends have direct repercussions for the contaminant dispersal in the coastal area. Whereas Pb and Zn conserve their enhanced levels because of their stronger affinity with fine sediments, Cu is marked by the entire loss of its anthropogenic component that is progressively transferred to the dissolved phase, likely mediated by organic ligands. We ascribe these behaviours to different post-depositional partition mechanisms with respect to oxidation of the particulate organic phase at the bottom sediment/water interface. Also, analysis of one sediment core from the prodelta indicates that these early diagenetic processes govern the chemical forms of land-derived contaminants sequestrated in the nearshore sedimentary archives. (C) 2011 Elsevier Ltd. All rights reserved.

Analysis of sediments in and around the large and deep burrows made by the callianassid shrimp Callichirus laurae, Jordan Gulf of Aqaba (Red Sea), showed that organic carbon (OC), humic matter (HM), humic (HA) and fulvic acids (FA) are redistributed both quantitatively and qualitatively by sediment reworking activities

Special issue EUROSTRATAFORM VOL. 1: Source to Sink Sedimentation on the European Margin.-- 14 pages, 8 figures, The importance of short-term processes, such as floods and storms, on sediment delivery and reworking on the Têt inner shelf was investigated. The Têt inner shelf is a small, event-dominated system located in the south-western part of the Gulf of Lions. The expected sedimentary scenario in this environment is that fluvial sediment should be deposited on the prodelta and later dispersed around the shelf and slope by wave and current reworking during storms. This paper investigates differences in inner shelf sediment dynamics between storm events occurring during usual river discharges and those occurring during river floods. Waves, current velocities, near-bottom water turbidity, bottom sediment grain-size and sea-floor erosion/accretion were concurrently measured at 28-m water depth on the Têt inner shelf from November 2003 to March 2004. Two major storms took place on 4 December 2003 and 21 February 2004, and two moderate storms occurred on 8 December 2003 and 14 March 2004. The two major storms displayed similar wave characteristics: maximum significant wave height (Hs)>7 m, peak period (Tp)>12 s and wave direction around 90°. The main environmental differences during the two major storm events correspond to the amount of sediment discharged from the Têt River. About 2 × 104 t of sediment was delivered by the river during the 4 December flood (wet storm) and less than 5 × 102 t during the 21 February storm (dry storm). Sediment dynamics were quite similar during the storm events: resuspension caused by waves and sediment advection towards the southeast due to near-bottom shelf currents were the dominant sedimentary processes. The result was a bottom sediment erosion of several centimetres at the study site during both the wet and dry events. The main differences between the wet and dry events arose after the storm. Immediately after the peak of the wet storm, sediment supplied by the Têt River (and probably from other rivers) was deposited around the river mouth. A few days later, during a moderate storm, this unconsolidated sediment was resuspended, transported offshore and deposited on the inner shelf. In contrast, a moderate storm which occurred some days after the dry February storm caused bottom erosion on the inner shelf, because no fresh sediment was available on the shallower area. The results from this study indicate that the Têt inner shelf at 28-m water depth is mainly a bypass zone for sediment that is transported S-SE towards the middle-outer shelf and slope, although ephemeral sediment deposits may be favoured by the sediment supplied from flood events. Sediment transport across the shelf, from the river to the slope, follows a complex, multi-step pattern that needs to be addressed using a multi-event approach., This study was supported by the EUROSTRATAFORMProject funded by the EU (EVK3-CT-2002–00079), within EU Fifth Framework Programme:Energy, Environment and Sustainable Development

Surficial sediments collected in 2002 throughout the Gulf of Lions continental shelf (NW Mediterranean) were analysed for trace metals (Cd, Co, Cr, Cs, Cu, Ni, Pb, Sr, Zn and Zr), major elements (Al, Ca, Fe, P and Ti) and the sewage marker coprostanol. In addition, particle size distribution, organic carbon (OC) and carbonates were also determined. Results showed that the metal contamination (Cd, P, Cu, Pb and Zn) is mainly introduced by the local rivers and accumulates—via a regulation by OC and silt fraction (2–63 μm)—in the direct vicinity of the mouths, in high shear stress environments. Here also the signal of sewage contamination is the best preserved, especially off the eastern point sources where local sedimentation rates save the faecal marker from biodegradation processes. It is demonstrated that the shallow prodeltas are the first repository areas for land-derived particles, exposing local ecosystems to both inorganic and organic contaminations. When going seaward, however, sediment dilution, particle sorting and biodegradation processes make that most riverborne contaminants rapidly reach natural levels. Only some metals (i.e. Pb and Zn)—closely associated with the clay fraction—still depict anthropogenic enrichment, which seems to be inherited from man-made aerosols.

The upper similar to50 cm sediments in the deep-sea Aegean Sea were generally deposited through hemipelagic rain as indicated by paucity of terrigenous sand-sized particles, abundance of clay-sized materials and lack of turbiditic beds. However, quartz sand-sized grains, black debris, higher C org content and ON ratio attest to a significant river-borne supply in the North Aegean Trough. Recurrent gravity-induced transfers from the shelf break are evidenced through various markers of coastal deposition dating back the last low-stand (Characea oogons, Elphidium crispum, Ammonia beccarii, glauconitic grains, evolved organic matter). Especially, this deposition induced significant rise of the C/N ratio of successive layers. The top of ash layers of the Minoan eruption was reached in several sample cores giving opportunity for an estimation of sedimentation rate. The rate values (7-15 cm per kyear) in the southern basin are not significantly lower than those (4-23 cm per kyear) in the northern basin in spite of a lower terrigenous supply. This is due to the abundance of cemented Mg-calcite clasts and nodules throughout the cores of the Cretan Basin and North Skiros Basin rising the carbonate content up to 60%. It is suggested that this high carbonate concentration participates in the relative preservation of the most labile components of the organic matter in a large southern part of the basin characterised by low C/N ratio (5.5-8). Around 2000-1200 year BP, thin darks layers were deposited and are not yet completely oxidised. They would attest to a very beginning stage of sapropelic sediment formation during times of stagnant bottom water probably related to the enhanced Black Sea water circulation., Les dépôts profonds récents de la Mer Égée résultent principalement de processus hémipélagiques comme l’indiquent la rareté de matériaux sableux terrigènes, l’abondance de la fraction fine argileuse et l’absence de lits turbiditiques. Cependant, dans la fosse nord-égéenne, la présence de grains grossiers, de débris noirs, les fortes teneurs en carbone organique et les valeurs élevées du rapport C/N indiquent des apports fluviatiles locaux. Plusieurs marqueurs côtiers du dernier bas-niveau marin (oogones de Characea, Elphidium crispum, Ammonia beccarii, grains glauconieux, matière organique évoluée) attestent de processus gravitaires récurrents issus du rebord externe du plateau. En particulier, ces flux gravitaires induisent une élévation significative du rapport C/N de plusieurs niveaux. Le toit des lits de cendre de l’éruption minoenne a été atteint ou traversé par la plupart de carottages permettant le calcul de la vitesse de sédimentation. Malgré un apport terrigène plus faible, les vitesses dans la partie sud du bassin (7–15 cm/kyr) sont du même ordre de grandeur que celles de la partie nord (4–23 cm/kyr). Cela résulte de l’abondance des clastes et des nodules de calcite magnésienne dans les carottes de la Fosse crétoise et du Bassin nord de Skyros qui se traduit par des teneurs en carbonates atteignant jusqu’à 60 %. Il est envisagé que ces fortes concentrations en carbonate favorisent la préservation des composés les plus labiles de la matière organique dans une large part du sud de la mer Égée caractérisée par de faibles rapports C/N (5,5–8). Entre 200 et 1200 BP, de fins lits sombres ont été déposés et ne sont pas encore complètement oxydés. Ils correspondraient à un stade initial d’accumulation de type sapropèlique pendant des épisodes de stagnation de l’eau profonde sans doute contrôlés par un renforcement de la circulation des eaux issues de la Mer Noire.

Downward particle fluxes off the Taotoi Pass, a typical Polynesian lagoon pass, have been measured over an annual cycle by means of an array of three mooring lines deployed at 40 m depth on the external sandy plain seawards of the Tiahura reef. Each line was equipped with two hand-operated sediment traps located 1.5 and 10 m above the bottom. During the 1-year survey, the traps were deployed for seven periods of 2-3 weeks. One line equipped with an automated trap was further deployed at 400 m depth on the deep fore-reef slope to collect deep settling particles over two consecutive 6-month periods. For all periods investigated, fluxes on the external plain were highest in front of the pass, and decreased by half on either side (up- and downstream with respect to general circulation), indicating a net export through the pass. At all three locations, the highest downward fluxes recorded (60-100 g m(-2) d(-1)) corresponded to coarse panicles mostly carbonate, whereas the lower fluxes (a few g m(-2) d(-1)) corresponded to smaller and lighter material mostly organic carbon. Fluxes on the deep slope were much lower than on the sandy plain (1.5-7.3 g m(-2) d(-1)), but followed a broadly similar seasonal trend. Comparison of our trap data with published production values for the Tiahura reef system suggests that inorganic and organic carbon export through the pass is limited. The minimum estimates represent around 6% of total calcification but only 1% of the net production of the reef system. The qualitative and quantitative temporal evolution of downward fluxes showed a seasonally-contrasted trend, related to local hydrodynamic conditions. Fluxes were always higher during the southern summer season, and lower during winter. Carbon export appears to be ultimately controlled by climatic factors. i.e. seasonal changes in the orientation and heights of swells and waves that modulate the amount of water flowing into the lagoon, before being flushed out into the open ocean, entraining particles. (C) 2002 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS. All rights reserved., Les flux particulaires ont été mesurés devant la passe lagonaire de Taotoï (côte Nord de Moorea, Polynésie française) sur une période annuelle, au moyen de trois lignes de mouillage de pièges à particules déployées à 40 m de profondeur sur la plaine sableuse située à l’extérieur du récif barrière de Tiahura. Chaque ligne comporte deux pièges à particules manuels situés à 1,5 m et 10 m au-dessus du fond. Ces pièges ont été déployés durant sept périodes de deux à trois semaines. Un piège à particules automatique a, en outre, été déployé à 400 m de profondeur sur le tombant afin de collecter, de façon séquentielle, des échantillons mensuels de particules pendant deux périodes consécutives de six mois. Les flux mesurés sur la plaine sableuse sont toujours plus élevés devant la passe. Ils diminuent de moitié environ pour les stations placées en « amont » et en « aval » de la passe (en référence au courant de dérive générale à l’extérieur du récif barrière), indiquant une exportation nette de matière par cette dernière. Les flux les plus élevés (60 à 100 g m–2 j–1) sont systématiquement composés de particules lourdes et carbonatées. À l’inverse, les flux les plus faibles (quelques g m–2 j–1) se composent dˈun matériel plus fin, plus léger, enrichi en carbone organique. En profondeur, les flux sont beaucoup plus faibles que sur la plaine sableuse (1,5 à 7,3 g m–2 j–1), mais présentent un signal saisonnier similaire. La comparaison de nos résultats avec les valeurs de production publiées pour le système récifal de Tiahura suggère que l’exportation de carbone inorganique et organique par le récif reste un phénomène limité, avec des minima dˈenviron 6% de la calcification totale et de 1% de la production nette. L’évolution temporelle des flux présente une tendance saisonnière marquée, liée à la variation des conditions hydrodynamiques locales: les flux sont toujours plus élevés en période d’été austral et plus faibles en hiver. L’exportation de carbone semble ainsi contrôlée par les facteurs climatiques, c’est-à-dire les changements saisonniers d’orientation des houles et des vagues qui modulent les arrivées d’eau dans le lagon et donc le transport particulaire associé au flux dˈeau sortant.

An analysis of sedimentological, sediment trap and hydrological data was performed to investigate the transport and fate of particulate matter in the Gulf of Lions. The sedimentological properties outline the major sedimentary units of the shelf (Rhone prodelta, mid-shelf mud belt, outer shelf) and slope. The geometry of these sedimentary units and the southwestwards increase of particulate fluxes on the slope highlight the influence of the general cyclonic circulation on the dispersion of land-derived particulate matter. Considering the known input and output terms, budgets of particulate matter and organic carbon in the Gulf of Lions are proposed. Inputs were river supply, atmospheric deposition and primary production; outputs were sediment burial and advective export on the slope. Degradation of particulate organic carbon in the water column and at the sediment-water interface was also estimated. Mass and POC budgets were balanced within uncertainties. Nevertheless, these results illustrate the difficulty in establishing budgets based on elements estimated at different time scales, for which the system is not necessarily in a steady state., Le dépôt et l'exportation de matière particulaire dans le golfe du Lion sont présentés au travers de résultats sédimentologiques, hydrologiques et de flux particulaires. Les principales unités sédimentaires sont mises en évidence sur le plateau (prodelta rhodanien, vasière médiane et plateau externe) et sur la pente. La déflexion des unités sédimentaires vers le sud-ouest et l'augmentation des flux particulaires actuels sur la pente occidentale soulignent le rôle essentiel de la circulation moyenne cyclonique sur la dispersion de matériel continental. En considérant les termes d'entrée et de sortie connus, des bilans massiques de matière particulaire et de carbone organique particulaire ont été établis. Les termes d'entrée sont les apports fluviatiles, les dépôts atmosphériques et la production phytoplanctonique. Les pertes du système sont le piégeage dans les sédiments et l'exportation vers le large (flux advectifs le long de la pente). La dégradation du carbone organique particulaire dans la colonne d'eau et à l'interface eau–sédiment est également estimée. Les bilans de masse et de COP apparaissent équilibrés compte tenu des incertitudes. Ces résultats illustrent néanmoins la difficulté à établir des bilans basés sur des éléments estimés à différentes échelles de temps, pour lesquelles le système n'est pas nécessairement stationnaire.

The distribution of global organic parameters (organic carbon, nitrogen, hydrolyzable carbon) was established in surficial sediments of the Gulf of Arzew. Moreover, humic substances, humin and hydrocarbons have provided information on the organic matter origin and evolution in the identified sedimentary areas: prodeltas at the mouth of the dry rivers Cheliff and Macta; muddy mid-deposits and carbonated shallow environments. The higher organic matter concentrations characterize the muddy mid-deposits and especially the western part located near the Arzew and Bethioua harbours (TOC > 0.9 %; Nt = 0.35 % d.w.). The marine origin of the organic matter is supported by low C/N ratios (5-9) in the total sediments and the position in the Van Krevelen diagram of the humic acid H/C, N/C and O/C atomic ratios. The predominant marine character results from an important phytoplanktonic production. Only organic matter of the Cheliff prodelta is characterized by a double marine and continental signal. The degradation state of organic matter shows that the compounds are more degraded in the western part of the gulf, towards the east to west transfer of matter. The Arzew and Bethioua harbours areas are enriched in hydrocarbons (90-180 mg kg(-1)) according to the N.S.A. norm (1975) and represent significant petrogenic pollution levels., La distribution des concentrations des paramètres organiques globaux (carbone organique, azote, carbone hydrolysable) a été établie pour l'ensemble des sédiments superficiels du golfe d'Arzew. L'étude des substances humiques, de l'humine et des hydrocarbures a permis de définir l'origine et le degré d'évolution de la matière organique dans les principales unités sédimentaires identifiées : zones prodeltaïques au droit des embouchures des principaux oueds (Chéliff et Macta), vasière centrale et hauts fonds carbonatés. La grande vasière située au centre du golfe est enrichie en matière organique ainsi que les zones portuaires d'Arzew et Béthioua (Corg > 0,9 % ; Nt = 0,35 % p.s.). Les rapports Full-size image (

International audience; he IGUANES cruise took place in May 2013 on the R/V L'Atalante along the Demerara passive transform margin off French Guiana and Surinam. Seismic, multibeam and chirp acquisitions were made. Piston cores were collected for pore geochemistry and sedimentology. A mooring was deployed on the sea-bottom for 10 months (temperature, salinity, turbidity and current measurements). This new dataset highlights the lateral variability of the 350 km-long Guiana–Surinam transform margin due to the presence of a releasing bend between two transform segments. The adjacent Demerara Plateau is affected by a 350 km-long giant slide complex. This complex initiated in Cretaceous times and was regularly reactivated until recent times. Since the Miocene, contourite processes seem to be active due to the onset of the North Atlantic Deep Water (NADW) bottom current. A main NADW water vein flows towards SE, eroding slide headscarps and allowing the deposition of contourite drifts. Numerous depressions looking like comet tails or comet scours record this flow. Some of those were interpreted before the cruise as active pockmarks. Pore geochemistry and core analysis do not show any evidence of present-day gas seepage.